Selective catalytic reduction (SCR) systems are sometimes employed with compression-ignition engines to reduce nitrogen oxides in the exhaust stream. SCR systems require the use of a reductant, such as anhydrous ammonia, aqueous ammonia, or urea in the exhaust stream. A control system regulates the amount of reductant injected into the exhaust stream based on the levels of oxides of nitrogen (NOx) in the exhaust stream. Typically, the control system employs a sensor that measures the levels of NOx in the exhaust stream to determine how much reductant to inject.